1. Field of the Invention
This invention relates to pneumatic radial-ply tires for motor vehicles, more particularly to a steel cord reinforcement therefor.
2. Prior Art
Advanced pneumatic radial-ply tires for passenger cars as well as heavy-duty automotive vehicles are provided typically with belt layers made up of rubber coated steel cords. Organic fiber cords such as of aramid have also found wide application for the belts of the radial tire but have mostly been accompanied with the use of rubber coated steel cords at least for one belt layer to provide increased rigidity.
While steel cords are suitable for use as belt reinforcement material in view of their high rigidity and strength, the cords being steel are susceptible to corrosion attack by water or moisture permeating through an external injury on the tire which would lead to separation from a coat rubber surrounding the cords and thus pose a durability problem. To solve this problem, there has been proposed a so-called rubber penetrated steel cord having two filaments in the tire core and a strand construction of 2+7, 2+6 and so on having increased filament-to-filament spacing in the outer sheath of the tire. However, such cords literally require so many steel wires that the belt portion of the tire becomes objectionably thick and that this is economically disadvantageous. Cords with less filaments such as of a 2+2 strand were also proposed and put to use. Even simpler cord structures having a 1.times.2 strand are also known as disclosed, for instance in Japanese Laid-Open Patent Publication Nos. 62-117893 and 62-234921. Coat rubber for 1.times.2 steel cords is desirably low in Mooney viscosity a the time of vulcanization to facilitate filling between the filaments of the cord, and preferably high in modulus after vulcanization to retain flexibility of the cord. These cords, however, are not quite satisfactory in terms of durability as the are also prone to get corroded by water permeation through the belt layers to which the cords are applied.